Qualities such as a texture, a feel and ease in turning the page are important for bookpaper. Particularly recently, paper qualities such as lightness in spite of heftiness (i.e., tall paper thickness), i.e., bulkiness (low density), and ease in turning the pages when the paper is used for books are demanded. In the past, if paper thickness is increased, stiffness is increased, making it harder to turn the pages. For this reason it was difficult to manage to have both paper thickness and ease in turning the pages.
Generally, qualities such as a feel and ease in turning the pages are the factors that are affected by the suppleness of paper. It is, however, difficult to digitize the suppleness of paper all of a lump, because body, elasticity, strength and other qualities are associated with it. With the aim of improving the texture of paper used as bookpaper, in Japanese Patent Laid-open No.1996-246390, thin bookpaper for which specific spindle-shaped calcium carbonate is used as filler and in which mechanical pulp with a water-retention value of 100 to 150% is compounded, has been disclosed. In Japanese Patent Laid-open No.1998-204790, low-density bookpaper of 0.6 to 0.65 g/cm3 in density, which contains 90 weight % or more hardwood kraft pulp (Freeness: CSF 500 ml or more) which the hardwood kraft pulp contains 50 to 100 weight % dipterocarp pulp, and which contains calcium carbonate as filler, has been disclosed. As for these bookpapers, however, because it was necessary to compound special pulp, they had a disadvantage cost-wise, and they fell short of suppleness and were not superior in texture and ease in turning the pages.
As environmental conservation is gaining momentum, producing lighter paper becomes a subject that cannot be avoided in terms of effectively utilizing paper pulp, which is produced from forest resources. Producing lighter paper has become a big current as well from the viewpoint of quality requirements for the bookpaper as mentioned in the above. Here, lighter paper implies reducing the weight of paper while maintaining the thickness of the paper, i.e. producing low-density (and bulky) paper.
As a method of lowering the density (increasing the bulkiness) of paper, paper pulp, which is a main raw ingredient of the paper, can be first examined. For the paper pulp, wood pulp is normally used. As pulp for reducing the density of paper, because of their stiff fibers, mechanical pulp such as ground pulp, which is obtained by grinding wood by a grinder without using a chemical, or thermo-mechanical pulp, which is obtained by fibrillating wood by a refiner, is more effective for lowering the density than chemical pulp, which is obtained by a chemical extraction of lignin, a reinforcing material contained in fibers. Particularly, ground pulp contributes largely to lowering the density. However, ground pulp, which is mechanical pulp, has a problem in compounding it in wood-free paper from the viewpoint of meeting the standards. Additionally, if it is compounded in the paper, there is a problem in paper quality (e.g., color reversion) with time. Consequently, compounding the ground pulp is impossible. By the same token, it is impossible to compound thermo-mechanical pulp as well.
In the case of wood-free paper, as for pulp, only chemical pulp can be compounded. By compounding chemical pulp, paper density is substantially affected by a pulpified wood type. In other words, the rougher and larger wood fibers themselves are, the easier lowering the density is possible. For wood-free paper, mainly hardwood pulp is compounded. Of the hardwood types, gumwood, maple and birch can be mentioned as wood types that can be used for lowering paper density. In a rise of the current environmental conservation trend, however, it is difficult to collect only these wood types by specifying them for pulping.
Mechanical paper or wood-containing paper, in which mechanical pulp is compounded, is a normally lower density paper than wood-free paper. Compounding stiff fibers causes picking (many of such incidents are caused by twined fibers derived from mechanical pulp) and lowers strength. Furthermore, because degree of brightness is degraded by increasing a compounding ratio of mechanical pulp whose degree of whiteness is lower than that of bleached chemical pulp, a compounding amount of mechanical pulp is restricted. With a recent growing tendency for environmental conservation and because of the need for protecting resources, increasing a compounding amount of recycled waste-paper pulp is called on. It is unlikely that recycled waste-paper pulp is pulpified by specifically grouping them according to paper quality types such as wood-free paper, newsprint paper, magazine paper, flyer paper, coated paper, etc. Instead, all different types of recycled waste paper are pulpified just as mixed. As a result, in terms of pulp qualities, density tends to become higher than that of virgin mechanical pulp. The reason for this is that fibers contained in recycled waster-paper pulp comprise a mixture of chemical pulp and mechanical pulp. Because talc, kaolin, clay and calcium carbonate, which are normally used as filler contained in paper or pigment for a coating layer of coating paper, have a higher density when compared with pulp, by compounding them, the density of paper tends to increase. Hence, increasing a compounding ratio of recycled waste-paper pulp tends to increase paper density.
As mentioned above, if taking the current status of wood resources and quality design of paper into consideration, it is very difficult to achieve sufficiently low density needed for the paper only from a pulp aspect.
Normally, for paper pulp, fibers are made supple by a refining process and are then fibrillated. Because bulkiness tends to decrease by the refining process, it is desirable not to perform refining process as much as possible for the purpose of increasing bulkiness. However, if the refining process is insufficient, strength decreases.
As a method for lowering density when making paper, pressure applied by a press should be brought down as much as possible during press process, and calendaring, which is performed to provide smoothness on the paper surface, should not be performed. Furthermore, it is desirable to use as little as possible of a coating amount for surface coating of water-soluble polymer such as starch. This coating is performed to provide the surface strength of paper when being printed.
In addition to applying some means at the time of pulping and making paper, fillers, which are compounded at a higher compounding ratio next to the pulp, have also been examined. For example, a method for achieving lower density by compounding hollow synthetic organic matter capsules as fillers has been disclosed in Japanese Patent Laid-open No.1993-339898. Synthetic organic expandable filler (e.g., a product name such as EXPANSEL manufactured by Nihon Filight), which achieves higher bulkiness by expanding by heat from a dryer portion of a paper machine, has been proposed. The method using these synthetic organic expandable fillers, however, has such problems that setting drying conditions is difficult, surface strength is weak and printing glossiness is lowered.
In Japanese Patent Publication No.1977-39924, a method using Shirasu-balloons has been proposed. In this method, there are such problems that they cannot be compounded in paper pulp well, and uneven printing results occur using the paper made in this method.
In Japanese Patent Laid-open No.1996-13380, a method for adding microscopic fibrillated cellulose has been disclosed. With this method, microscopic fibrillated cellulose has to be fabricated specially. At the time making paper, it is necessary to adjust freeness of pulp to CSF 400 ml or more, preferably to CSF 500 ml or more. It is difficult to adjust freeness for paper stock in which mechanical pulp is compounded at a high compounding ratio; therefore, it is difficult to use this method for making mechanical paper and wood-containing paper.
Using the above-mentioned methods, paper thickness was increased. However, because the paper thickness was increased, the stiffness of paper rose exponentially and suppleness of the paper was not improved. As a result, texture, feel and ease in turning the page were not sufficient.